In a gas turbine aircraft engine air enters at the engine inlet and flows from there into the compressor. Compressed air flows to the combustor where it is mixed with fuel injected via fuel nozzles and the fuel-air mixture is ignited. The hot combustion gases flow through the turbine. The turbine extracts energy from the hot gases, converting it to power to drive the compressor and any mechanical load connected to the drive.
The fuel is supplied from the fuel tank to the fuel nozzles through the fuel manifold, and thereby to the combustor, via the fuel system piping. It is conventional practice to shroud the connection between the fuel system piping and the fuel nozzles. Fuel shroud systems are also used in conjunction with fuel system piping connected to fuel-actuated components, such as the variable stator vane system, the variable bleed valve system and the turbine clearance control system.
The basic purpose of such fuel shroud systems is to provide a drainage path for fuel in the event of a fuel leak at or near the connection of the fuel system piping to the component interface. The fuel shroud system is designed to prevent the leaked fuel from dripping down onto hot surfaces in the engine compartment, which condition is conducive to undesirable combustion.
A conventional fuel shroud system of the type shown in FIG. 1 has two main components: a drain can subsystem, generally designated by numeral 2, and a sealing ferrule 4.
The drain can subsystem includes a drain can nut 2a and a drain can 2b. Drain can nut 2a comprises a substantially cylindrical portion 6 and an annular flange portion 8 projecting radially inward at one end of the cylindrical portion 6. Drain can 2b comprises a substantially cylindrical portion 10 and an annular flange portion 12 projecting radially outward at one end of the cylindrical portion 10. Flange portion 12 has a radial sealing surface 18a. The outer diameter of cylindrical portion 10 of drain can 2b is less than the inner diameter of flange portion 8 of drain can nut 2a, allowing cylindrical portion 10 to pass through the opening defined by flange portion 8. The outer diameter of flange portion 12 of drain can 2b is greater than the inner diameter of flange portion 8 of drain can nut 2a, blocking passage of flange portion 12 through the opening defined by flange portion 8. The exposed surfaces of drain can nut 2a and drain can 2b can be coated with any conventional heat shielding material to prevent overheating of the fuel flowing through sealing ferrule 4.
The drain can 2b is securely mounted on the component interface 14 by way of drain can nut 2a. The inner circumferential surface of cylindrical portion 6 of drain can nut 2a is provided with a thread 16a for coupling with a thread 16b formed on an outer circumferential surface of the component interface 14. When drain can nut 2a is tightened onto the component interface 14, the flange portion 8 of drain can nut 2a clamps the sealing surface 18a of flange portion 12 of drain can 2b against a radial sealing surface 18b of the component interface 14.
A connector nipple 24 is tightly screwed into a threaded bore in interface component 14. The connector nipple may be made of steel while the interface component may be aluminum. The component interface 14 and connector nipple 24, which form the interface component subsystem, each have a bore of predetermined diameter. The bores of component interface 14 and connector nipple 24 communicate to form a fluid channel 20 for carrying fuel.
The sealing ferrule 4 is securely coupled to connector nipple 24 by a swivel nut 26. Sealing ferrule 4 has a fluid channel 22 for carrying fuel from fuel pipe 27. Sealing ferrule 4 and swivel nut 26 form a fuel piping subsystem.
Swivel nut 26 comprises a substantially cylindrical portion 28 and an annular flange portion 30 projecting radially inward at one end of the cylindrical portion 28. The inner circumferential surface of cylindrical portion 28 of swivel nut 26 is provided with a thread 32a for coupling with a thread 32b formed on an outer circumferential surface of the connector nipple 24.
As the swivel nut 26 is screwed onto the connector nipple 24, a sealing surface 34a at one end of connector nipple 24 comes into sealing contact with a corresponding sealing surface 34b formed on one end of the sealing ferrule 4. Sealing surfaces 34a and 34b form a contact seal which prevents leakage of fuel when channel 20 is in fluid communication with channel 22. In accordance with conventional practice, one of the sealing surfaces 34a and 34b can be frusto-conical while the other is generated by rotation of an arc about the axis of symmetry. In particular, sealing surface 34b can be frusto-conical while sealing surface 34a is a spherical section.
The sealing ferrule 4 in accordance with conventional practice comprises three pieces welded together: a tube 36, a connector 38 butt-welded at joint 40 to one end of tube 36, and a bushing 42 brazed at joint 44 to the outer circumferential surface of tube 36. The bushing 42 fits snugly inside the cylindrical portion 10 of drain can 2b to close one end of the annular sealed chamber 46 formed inside drain can 2b.
The outer circumferential surface of bushing 42 forms a sealing surface 48a which contacts an opposing sealing surface 48b formed by the inner circumferential surface of cylindrical portion 10 of drain can 2b. An O-ring 50 is seated in an annular recess formed on sealing surface 50a of bushing 42. O-ring 50 seals the sealed chamber 46 at the interface of sealing surfaces 48a and 48b. Similarly, O-ring 52, which is seated in an annular recess formed on the sealing surface 18a of drain can 2b, seals the sealed chamber 46 at the interface of sealing surfaces 18a and 18b.
Any fuel which leaks through the connector nipple/sealing ferrule contact seal 34 is contained inside the sealed chamber 46. The contained fuel in sealed chamber 46 is drained off to a safe environment via a drain outlet and a drain manifold (not shown in FIG. 1) in a well-known manner.
In the conventional fuel shroud system, a retainer ring 54 is seated in an annular groove 56 formed in the inner circumferential surface of drain can 2b. Retainer ring 54 has an inner diameter which is less than the outer diameter of bushing 42. Thus, retainer ring 54 prevents the sealing ferrule 4 from disengaging the drain can subsystem 2 in the event that swivel nut 26 were to loosen during operation.
The above-described conventional fuel shroud system suffers from several disadvantages.
First, in some applications it is necessary for installation of the drain can that the drain can be rotatable relative to sealing ferrule about an axis of rotation which is transverse to the longitudinal axis of the sealing ferrule. In the conventional fuel shroud system, such rotation is blocked by the outer circumferential sealing surface 48a of the bushing 42.
Second, cracks can form in the weld between tube 36 and connector 38 of sealing ferrule 4. Such cracks can lead to fuel leakage at the tube/connector interface, thereby subverting the integrity and reliability of the fuel piping subsystem.
Third, in those applications where a hose section separates the piping subsystem formed by the swivel nut and sealing ferrule from the section of piping which has rigid connection to the engine, the hose section can become torsionally preloaded when the swivel nut is torqued while being installed on the engine. Such torsional preloading has the potential to cause failure of the hose section. This is also true, but to a lesser degree, where the hose section is replaced by rigid piping from the swivel nut to the mounting connection on the engine. This type of torsional loading can contribute to failure of the rigid piping during operation.